Locomotives utilizing multiple engines are known. Multi-engine electrical connections and control methods are described, for example, in U.S. Pat. Nos. 7,304,445; 7,667,347 and 8,220,572 (each incorporated by reference in entirety).
In single engine locomotives using DC traction motors, the speed of the engine and the excitation of the engine's alternator are typically used to control the voltage across the traction motors so that, upon locomotive start-up, the current through the traction motors can be maintained within acceptable limits until a back-emf develops with increasing traction motor speed.
FIG. 1 illustrates a DC traction motor current as a function of locomotive speed. The current is initially very high because of the low electrical resistance of the traction motors. As the speed of the locomotive increases, a back-emf is developed within the traction motor which reduces the current through the traction motor as locomotive speed increases.
FIG. 2 illustrates a DC traction motor back-emf as function of traction motor field current. The back-emf is expressed as volts per traction motor rpm.
In the early 2000s, hybrid locomotives were developed whereby a small engine was combined with a large battery pack such as described in U.S. Pat. No. 6,308,639 and U.S. Pat. No. 6,812,656 (each incorporated by reference in entirety) for example. In these locomotives, the battery pack voltage when applied to a DC bus, clamps a large voltage across the traction motors thereby establishing too high a voltage, and hence too high a current, upon locomotive start-up. The solution to this problem was to use chopper circuits with each traction motor to control voltage across the traction motors as described in U.S. Pat. No. 6,812,656 and U.S. Pat. No. 6,984,946 (each incorporated by reference in entirety) for example.
Shortly after the introduction of the battery dominant hybrid locomotive, multi-engine locomotives were introduced. These multi-engine locomotives have come to be known as ‘genset’ locomotives. Initially these genset locomotives used two or more engines driving alternators with their rectifier networks connected electrically in parallel to a DC bus. The traction motor circuits connected to the DC bus retained the chopper circuits to control voltage across the traction motors. The chopper circuit voltage control system was also retained in disclosures of multi-engine locomotives with engines driving alternators with rectifiers connected in series (see FIG. 23 of U.S. Pat. No. 7,667,347, incorporated by reference in entirety). Retaining the chopper voltage control method has an advantage in control of wheel slip and the ability to deliver both high current and high voltage to the traction motors. However, such chopper control adds considerable expense and complexity to the point that these advantages are discounted because of the additional complexity especially in control and operation. The genset locomotives can now be viewed as a failed experiment judging by examples of customers acquiring single engine locomotives to replace their genset locomotives.
Genset locomotives typically use an algorithm linked to the throttle to manage engines in order to select a combination of generators to meet the desired overall locomotive power. Delays caused by engine starting and accelerating affects throttle response more and more with an increase in throttle. Conversely, needless running of an engine or engines, typically occurring by lowering the throttle, causes a time delay in shutting down this unneeded power. Software is used because there are a number of ways to balance engine outputs and locomotive response, depending on the objective (maximum power, fuel economy, engine life, emissions profile etcetera). This has led to some dissatisfaction by locomotive operators who are accustomed to having more operational control and more responsive control over locomotive power, tractive effort and speed.
Genset locomotives may also experience engine lifetime issues because of the practice of high-speed idle necessary to maintain sufficient DC bus voltage to operate the traction motor chopper circuits and to operate the auxiliary equipment.
The genset locomotive has been promoted as locomotive with low exhaust emissions. Areas like the Los Angeles with its unhealthy air-shed have deployed genset locomotives in an effort to reduce air pollution from locomotives. These genset locomotives are used mainly for switching in rail yards and cargo terminals. Genset locomotives are considered purpose-built switcher locomotives characterized by relatively low horsepower and low speed operation.
The reason genset locomotives have lower emissions than conventional locomotives is that they use multiple high-speed diesel engines (typically 2 to about 4) that are inherently cleaner than a single large, medium-speed locomotive diesel engine. Additionally the power requirement for the locomotive can be provided through the selection of engines as needed. Low speed movement of large group of rail cars may require high starting tractive effort (about 50,000 lbs or more) but, because of low speed operation (less than about 5 mph), a 600 HP engine is typically sufficient. Operating one 600 HP engine reduces both fuel consumption and emissions compared to operation of a 2,000 HP engine doing the same task. If the same group of cars must be accelerated to a speed higher than about 10 mph, as in the practice of “flat switching” and “kicking cars”, a second engine can be brought on-line.
Because the handling requirements of railcars in rail yards, cargo terminals and other industrial settings, deciding on the number of engines to engage can be separated into two regimes based on locomotive speed. At low speed, tractive effort is typically maximized. At high speed, engine voltage must be maintained to overcome the back emf. Having a multi-engine locomotive has merit. Having engines driving alternators with rectifiers whose output is electrically connected in series is a practical means of configuring the traction power circuit without going to the added cost and complexity of power electronics and associated requirements.
There is a need for a simplified method of electrically connecting the alternators with rectifiers for each engine of a multi-engine locomotive and a simplified method of controlling the sequencing and output of two or more engines.